Mt. Pinatubo's cloud shades global climate.
In a series of eruptions starting on June 15 of last year, Mt. Pinatubo ejected an estimated 20 million tons of sulfur dioxide gas into the stratosphere, where it formed tiny droplets of sulfuric acid (SN: 8/31/91, p.132). Such droplets, or aerosols, remain suspended in the upper atmosphere for several years following an eruption. Mt. Pinatubo's aerosol cloud, which circled the globe within a few weeks of its emergence, led scientists to speculate that incoming sunlight might be scattered or blocked, resulting in cooler temperatures on the ground.
Volcanic aerosols have now been caught in the act, the National Oceanic and Atmospheric Administration (NOAA) announced earlier this month.
Ellsworth G. Dutton, a meterologist with NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo., traced the effects of Pinatubo's cloud with ground-based instruments that directly measure the strength of sunlight. Dutton says his results show a 20 to 30 percent decline in the amount of solar radiation that reaches the ground without being scattered or reflected, and a 2 to 4 percent decline in total solar radiation.
Temperatures have already started to drop, both at ground level and in the lower atmosphere, says James K. Angell of NOAA in Silver Spring, Md. Angell told Science News his analyses of weather balloon data show that the first half of 1992 was 0.4 [degrees] C cooler, overall, than the first half of 1991. He notes that the volcano's effect may be greater than suggested by these observed temperature shifts, since this year's El Nino warming would normally raise average temperatures by 0.2 [degrees] C (SN: 1/18/92, p.37).
Weather satellites confirm cooling in the lower atmosphere, recording a global drop of more than 0.5 [degrees[ C since last June, with this June being 0.2 [degrees] C cooler than average, according to John Christy of the University of Alabama at Huntsville and Roy Spencer of NASA's Earth Science Lab at the Marshall Space Flight Center in Hunstsville. Christy says their data indicate that the greatest cooling, 1.0 [degrees] C, occurred in the northern midlatitudes -- an area that includes the continental United States -- while temperatures in the southern hemisphere have dropped by only 0.3 [degrees] C.
James E. Hansen, a climate modeler at NASA's Goddard Institute for Space Studies in New York City, eagerly follows such weather reports. His computer models of the volcano's effects have predicted that the upper atmosphere would warm as aerosol particles scattered and absorbed solar energy, while the partially shaded lower atmosphere would cool. Although some initial temperatures did not clearly follow Hansen's predictions, the recent reports fit well with his model.
According to Hansen's model, average surface temperatures will continue to drop, for a maximum cooling of 0.6 [degrees] C by the end of this year. Then, temperatures will gradually return to normal by 1994 as the volcanic aerosols slowly settle back down to Earth.